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Gills/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby Tim tests how long he can hold his breath underwater. Moby times him with a stopwatch. Tim jumps out of the water and Moby shows him how long he was underwater. MOBY: Beep. TIM: One minute? That's all? Moby shrugs. MOBY: Beep. Tim reads from a typed letter. TIM: Dear Tim and Moby, Can you explain how fish breathe underwater? From, Janice. Fish can breathe underwater because they have something called gills. Gills are like our lungs, they're organs for breathing, or respiration. An animation shows a fish and highlights its gills, behind its head. TIM: Respiration is how organisms take in oxygen and expel carbon dioxide. An animation shows a man breathe in oxygen molecules and breathe out carbon dioxide molecules. TIM: Besides fish, some species of worms, shellfish, and young amphibians use gills to breathe underwater. An image shows oxygen molecules floating around in water. TIM: They breathe the oxygen gas that's dissolved in water, just like we breathe in the oxygen gas that's spread out through the air. Getting oxygen out of water isn't very easy. Water has about one-fifth as much oxygen as air does, and it's also much heavier and thicker than air. An animation shows there are more oxygen molecules in air than water An image show different types of fish swimming in water. TIM: That means gills only really work for cold-blooded animals because their bodies require less oxygen to survive. Warm-blooded sea animals, like whales and dolphins, have lungs and breathe air just like we do. A whale jumps out of the pool and startles Moby. MOBY: Beep. An animation shows the flow of water through a fish's mouth and over its gills. TIM: Well, to get water flowing through their gills, fish take a big gulp of water, or just swim with their mouths open. The gills are mostly made up of thin pieces of tissue called lamellae. Inside the lamellae, is a network of arteries and tiny capillaries that bring blood really close to the surface. When water flows over the gills, oxygen from the water enters the blood. An animation shows the lamellae in detail and the flow of oxygen in the gills. MOBY: Beep. An animation shows molecules crowded on the left side of the screen spreading out evenly across the screen. TIM: Well, it works by a process called diffusion. That's the natural tendency of molecules to move from the area of high concentration to an area of low concentration. An animation shows a drop of food coloring dissolving in a glass of water. TIM: If you drop some food coloring into a glass of water, you can see diffusion at work. The blood entering the fish's lamellae contains less oxygen than the surrounding water. To even out the concentration, oxygen from the water enters the blood through the thin capillary walls. At the same time, the extra carbon dioxide in the fish's blood will diffuse into the water. To make the exchange of oxygen and carbon dioxide even more efficient, a lot of fish have an adaptation called a countercurrent exchange. Blood flows one way through the capillaries and water flows the opposite way across the lamellae. That way, as the blood flows through the gills, it always has slightly less oxygen than the water next to it. So oxygen flows into the fish's blood continuously. An animation shows how oxygen from the water, flowing to the left, enters the blood, flowing to the right, through capillary walls. Then carbon dioxide goes from the cells back into the water. TIM: Once the blood has gotten all the oxygen it can hold, it passes back through the body, and eventually makes its way back to the fish's heart. From the heart, blood gets pumped out to the gills again to pick up more oxygen. An animation shows a diagram of a fish, with the blood circulating through a fish's gills and heart. MOBY: Beep. TIM: Lamellae have evolved to have as much surface area as possible. The bigger the surface area, the more blood comes into contact with water, and the more oxygen the animal can get. In some fast-moving fish, the surface area of the gills is ten times larger than the surface area of the entire animal. An image shows the grooved surface of the lamellae. Moby gives Tim the stopwatch. MOBY: Beep. TIM: I'm not timing you. You don't breathe. MOBY: Beep. TIM: Ah. Tim holds the stopwatch. It reads 14:59 with the seconds moving up. TIM: Are you done yet? Moby, underwater, shakes his head no. Category:BrainPOP Transcripts